The invention relates generally to spectroscopy methods and apparatuses, and more particularly to a method of calibrating a wavelength-modulation spectroscopy apparatus.
The majority of wavelength-modulation spectroscopy gas analyzers employ a technique called 2f, where laser wavelength is modulated at 1f but the resulting signal demodulated at the second harmonic, designated as 2f, to produce the second harmonic spectrum of the analyte. The magnitude of the harmonic spectrum (e.g., the peak height) is then used to determine the analyte concentration in the sample gas, such as, but not limited to, the concentration of moisture (water vapor) in natural gas. This determination of analyte concentration is made based on a defined relationship between the analyte concentration and the magnitude of the harmonic spectrum, and often relies on the peak height of second harmonic spectrum. However, the 2f peak height is affected not only by analyte concentration, but also by the pressure, temperature, and composition of the sample gas. As a result, calibration of these analyzers is sample gas-specific (i.e., an analyzer calibrated to measure the concentration of an analyte in sample gas A (e.g., natural gas) cannot accurately measure the concentration of the same analyte in sample gas B (e.g., nitrogen or air)). Furthermore, an analyzer designed to determine the concentration of an analyte in a particular sample gas that is hazardous (e.g., toxic, flammable, explosive, etc.) can only be calibrated during manufacturing and re-calibrated using the same hazardous gas. Therefore, a need exists for a method of calibrating a wavelength-modulation spectroscopy apparatus that would allow for calibration with a relatively safe gas while providing the ability to measure analyte concentration in various sample gases.